A scanning device, such as a scanning electron microscope or a television camera, on scanning an object produces a number of signals which relate to characteristics of the object, together with synchronising and blanking signals. The synchronising signals are issued at the beginning of each line scanned and the blanking signals issue at the end of each line scanned. Vertical synchronising and blanking signals issue at the beginning and end, respectively, of each field scanned. The scan which the scanning device performs is known as a raster scan. In particular, with respect to a scanning electron microscope, signals can be generated by detectors relating to the detection of secondary electrons generated from the scanned object and backscattered electrons both pursuant to the incidence of a scanning beam of the electron microscope on the surface of the object. As discussed in the specification of Australian Patent No. 549,193 it is desirable to provide means which processes, in real-time, the signals produced by the detectors of a scanning electron microscope so that the processed signals may be used in directly driving a colour monitor. Thus characteristics of the scanned object at a particular location thereon can be rapidly determined using the display produced on the colour monitor by the processed signals, which are representative of the characteristics. Also it is desirable to provide real-time processing means which enables the signals used to drive the colour monitor to each be derived from one or more of the signals generated by a scanning device.